Crystal Structure of the Outer Membrane Protein - Toc34 from Chloroplasts


Chwan-Deng Hsiao1, Yuh-Ju Sun1,2, Farhad Forouhar1, Hsou-min Li1, and Chia-Cheng Chou1 1Institute of Molecular Biology, Academia Sinica, Nankang, Taipei, Taiwan 2Department of Life Science, National Tsing Hua University, Hsinchu, Taiwan


          Toc34 (34-kDa Translocon at the outer envelope membrane of chloroplast) is a member of the outer membrane translocon complex that mediates the initial stage of protein import into chloroplasts.  Toc34 is proposed to play a regulatory role that regulates the gating properties of Toc75 or regulate the recognition and presentation of precursor proteins. It is integrated into the membrane in a Nout-Cin orientation with a 29-kDa GTP-binding domain exposed to the cytoplasm, a single transmembrane domain, and a predicted 3-kDa C-terminal tail exposed to the intermembrane space of the envelope.  The structure information of the Toc components will provide better understanding of how the Toc complex functions to mediate precursor protein import.  It may also provide insight into how a possible regulatory component, like Toc34,  exert its regulatory function. We report here the three-dimensional structure of Toc34 in its GDP-bound.  In addition, a unique C-terminal a-helix provides a structural basis for the Toc34 protein anchor into outer membrane.